1. Field of the Invention
The present invention is directed to a cardiac pacemaker, and in particular to a cardiac pacemaker wherein the stimulation interval between stimulation pulses is adjustable.
2. Description of the Prior Art
A generally known cardiac pacemaker is the so-called QT-or stimulus-T pacemaker such as is described for example in U.S. Pat. No. 4,228,803. Such a pacemaker has means with which the median stimulation frequency can be adapted to changes in physical and psychic stress.
To this end a circuit is provided which evaluates the ECG signal derived intracardially, detecting the beginning or the maximum of the T wave. Since the time interval between stimulation and the start of the T wave, the so-called stim-T interval shortens with increasing stress, the circuit delivers a physiological measuring parameter with which the stimulation frequency can be adapted to changing stresses.
The principle disadvantage of a frequency control system of this kind is due to the fact that the stim-T interval does not shorten only with an increase in stress, but shortens to a considerably greater degree through the rise of the stimulation frequency itself. Frequency control of this type correspondingly requires special measures in order to avoid positive feedback.
A further disadvantage of this system of frequency control is the fact that the measured stim-T intervals are dependent on hormonal secretions, i.e., they respond to hormones secreted by the adrenal cortex and transported via the blood circulation.
In principle, in the regulation of the stimulation frequency in cardiac pacemakers it is an essential goal to adapt the stimulation frequency not only to rising physical stresses, but also to take into account the individual myocardial capacity of the patient. This means that the stimulation frequency is increased with rising stress only as long as a rise in the heart time volume (HTV) is achieved. This is intended to prevent the myocardium from being overloaded and damaged by too high a stimulation frequency (“overpacing”).
An attempt has been made to achieve this control by measuring the beat volume BV or an HTV-dependent measuring parameter, such as for example the central venous oxygenation (s02).
From PCT Application WO 89/06990 a method is known for hemodynamic optimization of the stimulation frequency, which uses the measurement of the central venous oxygenation s02, dependent on the heart time volume, in combination with a modulation of the stimulation frequency ΔHR over phases of two to four minutes. Optimization of the heart time volume is sought in that the frequency-dependent gradient of the oxygenation AsO2/ΔHR is kept within a predetermined range, which is a physiologically optimum range analogous to the gradient of the heart time volume HTV/ΔHR.
This method depends on the stability and the accuracy of the s02 sensor catheter, which in practice have not proved to be sufficient, and the method has the disadvantage that on account of the necessary long change periods it is not possible in the necessary time of a few minutes to differentiate whether the measured s02 change is caused by the frequency change or by other influencing variables.
European Application 0 551 355 describes a method for modulating individual stimulation intervals in which the impedance measurement is used to detect the beat volume, in order to avoid the use of a sensor catheter to determine the heart time volume. Through the deliberate modulation of individual stimulation intervals ΔSI and the phase-specific demodulation of the impedance change ΔZ, an attempt was made to suppress the influence of non-function-specific and thus disturbing parameter changes, and in addition the signal was calibrated with the aid of maximum modulation.
This method has the disadvantage that the principle of modulating individual stimulation intervals here is only used as a filtering and calibration method, i.e. as an interim step to determine the beat volume and thus the heart time volume (HTV). Optimization of the frequency control is then also sought by the optimization of the gradient ΔHTV/ΔHR on the basis of an optimum hemodynamic characteristic curve. The determination of the beat volume, despite an improvement in the signal-to-noise-ratio as a result of the individual pulse modulation, has in practice still proved too inaccurate to be able to carry out reliable hemodynamic optimization. This means that optimization of the stimulation by controlling the heart time volume has in practice been problematic, since either the specific sensor catheters for measuring the beat volume or the HTV-dependent measuring parameters still have no adequate long-term stability, or measurements of the beat volume using standard catheters via the impedance are not sufficiently reliable. Moreover the evaluation becomes very complex since the mechanical transmission functions also detected and which falsify the measuring result must also be taken into account.